A diffractive optical element is structured so that a diffraction grating for diffracting light is provided on a body which is composed of an optical material such as a glass or a resin. Diffractive optical elements are used in the optical systems of various optical devices, including imaging devices and optical recording apparatuses. For example, lenses which are designed to gather diffracted light of a specific order to one point, spatial low-pass filters, polarizing holograms, and the like are known.
A diffractive optical element has an advantage in that it allows for a compact optical system. Moreover, conversely to refraction, a greater diffraction occurs for light of longer wavelengths. Therefore, by combining a diffractive optical element and a usual optical element which utilizes refraction, it is possible to improve the chromatic aberration and curvature of field of an optical system.
However, since diffraction efficiency theoretically depends on light wavelength, there is a problem in that, if a diffractive optical element is designed so as to attain an optimum diffraction efficiency for light of a specific wavelength, its diffraction efficiency will be lower for light of any other wavelength. For example, in the case where a diffractive optical element is employed in an optical system which utilizes white light, e.g., a lens for a camera, such wavelength dependence of diffraction efficiency will cause uneven color and flares due to light of unwanted orders, and thus it is difficult to construct an optical system having appropriate optical characteristics with diffractive optical elements alone.
Against such problems, Patent Document 1 discloses a method of constructing a phase-difference type diffractive optical element, where a diffraction grating is provided on the surface of a body that is composed of an optical material, the diffraction grating being covered with an optical adjustment layer which is composed of an optical material different from that of the body, and by selecting two optical materials so that their optical characteristics satisfy predetermined conditions, the diffraction efficiency at a designed order of diffraction is increased regardless of the wavelength; that is, the wavelength dependence of diffraction efficiency is reduced.
Assuming that the light which is transmitted through the diffractive optical element has a wavelength λ; refractive indices of the two types of optical materials at the wavelength λ, are n1(λ) and n2(λ); and the diffraction grating has a depth d, then the diffraction efficiency with respect to light of the wavelength λ, will be 100% when eq. (1) below is satisfied.
                    [                  Eq          .                                          ⁢          1                ]                                                            d        =                  λ                                                                n                ⁢                                                                  ⁢                1                ⁢                                  (                  λ                  )                                            -                              n                ⁢                                                                  ⁢                2                ⁢                                  (                  λ                  )                                                                                                    (        1        )            
Therefore, in order to reduce the wavelength dependence of diffraction efficiency, an optical material having a refractive index n1(λ) and an optical material having a refractive index n2(λ) may be combined which have wavelength dependences such that d is approximately constant in the wavelength band of the light used. Generally speaking, a material having a high refractive index and a low wavelength dispersion and a material having a low refractive index and a high wavelength dispersion are to be combined. Patent Document 1 discloses using a glass or a resin as a first optical material to become the body and using a UV-curing resin as a second optical material.
When glass is used as the first optical material to become the body, micromachining becomes more difficult than in the case of a resin, and it is not easy to obtain a narrow diffraction grating pitch for improving the diffraction performance. Thus, it is difficult to enhance the optical performance while downsizing the optical element. Moreover, since the molding temperature of glass is higher than that of resin, a mold for molding glass has a lower durability than that of a mold for molding resin, thus resulting in a producibility problem.
On the other hand, when a resin is used as the first optical material to become the body, the diffraction grating has a better processability and moldability than those of glass. However, it is more difficult to realize various refractive index values than in the case of glass, so that the reduced refractive index difference between the first optical material and the second optical material is reduced, and thus the diffraction grating will have a large depth d, as is clear from eq. (1).
As a result of this, although the body itself has an excellent processability, the mold must be processed deep for forming the diffraction grating and the edges of the grooves must be shaped sharp, which makes the mold processing difficult. Moreover, due to processing constraints of at least one of the body and the mold, the pitch of the diffraction grating need to be made larger as the diffraction grating becomes deeper. Therefore, the diffraction grating cannot be increased in number, and the design constraints of the diffractive optical element are increased.
In order to solve such problems, the Applicants of the present application have proposed in Patent Document 2 to use a composite material as an optical adjustment layer, such that inorganic particles with an average particle size of 1 nm to 100 nm are contained in a matrix resin. With this composite material, it is possible to control the refractive index and Abbe number depending on the material of the inorganic particles to be dispersed and the added amount of the inorganic particles, thus providing refractive indices and Abbe numbers beyond what can be obtained with conventional resins. Therefore, by employing a composite material for the optical adjustment layer, an increased design freedom for the diffraction grating is obtained when using a resin as the first optical material to become the body, and an improved moldability is obtained, while providing wavelength characteristics with an excellent diffraction efficiency.